Computer Room Air Conditioner With Pre-Cooler

ABSTRACT

A computer room air conditioner (“CRAC”) has a cabinet having an air inlet through which return air from an area is drawn and an air outlet through which air cooled by the CRAC is exhausted. An air moving unit is disposed in the cabinet as are a plurality of cooling coils, which are in separate cooling circuits. The cooling coils are arranged so that the air passes through the cooling coils in serial fashion, that is, first through an upstream cooling coil and then through a downstream cooling cool. The upstream cooling coil acts as a pre-cooler to the subsequent downstream cooling coil. The CRAC includes a controller that controls the cooling provided by the cooling circuits. The controller controls the cooling provided by the upstream cooling circuit so that when it is being used to provide cooling, it provides only sensible cooling.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of U.S. Provisional Application No.61/346,951 filed on May 21, 2010. The entire disclosure of the aboveapplication is incorporated herein by reference.

FIELD

The present disclosure relates to data centers and data center coolingsystems, such as data center cooling systems having computer room airconditioners.

BACKGROUND

This section provides background information related to the presentdisclosure which is not necessarily prior art.

A data center is a room containing a collection of electronic equipment,such as computer servers. Data centers and the equipment containedtherein typically have optimal environmental operating conditions,temperature and humidity in particular. A climate control system isutilized to maintain the proper temperature and humidity in the datacenter.

FIG. 1 shows an example of a typical data center 100 having a climatecontrol system 102. Data center 100 illustratively utilizes the “hot”and “cold” aisle approach where equipment racks 104 are arranged tocreate hot aisles 106 and cold aisles 108. Data center 100 is alsoillustratively a raised floor data center having a raised floor 110above a sub-floor 112. The space between raised floor 110 and sub-floor112 provides a supply air plenum 114 for conditioned supply air(sometimes referred to as “cold” air) flowing from computer room airconditioners (“CRACs”) 116 of climate control system 102 up throughvents 113 (only one of which is shown in FIG. 1) in raised floor 112into data center 100. The conditioned supply air then flows into thefronts of equipment racks 104, through the equipment (not shown) mountedin the equipment racks where it cools the equipment, and the hot air isthen exhausted out through the backs of equipment racks 104.

It should be understood that data center 100 may not have a raised floor110 nor plenum 114. In this case, the CRAC's 116 would draw in throughan air inlet (not shown) heated air from the data center, cool it, andexhaust it from an air outlet 117 shown in phantom in FIG. 1 back intothe data center. The CRACS 116 may, for example, be arranged in the rowsof the electronic equipment, may be disposed with their cool air supplyfacing respective cold aisles, or be disposed along walls of the datacenter.

In the example data center 100 shown in FIG. 1, data center 100 has adropped ceiling 118 where the space between dropped ceiling 118 andceiling 120 provides a hot air plenum 122 into which the hot airexhausted from equipment racks 104 is drawn through vents 123 (only oneof which is shown in FIG. 1) in ceiling 118 and through which the hotair flows back to CRACs 116.

CRACs 116 may be chilled water CRACs or direct expansion (DX) CRACs.CRACs 116 are coupled to a heat rejection device 124 that providescooled liquid to CRACs 116. Heat rejection device 124 is a device thattransfers heat from the return fluid from CRACs 116 to a cooler medium,such as outside ambient air. Heat rejection device 124 may include airor liquid cooled heat exchangers. Heat rejection device 124 may be abuilding chilled water system in which case chilled water is the cooledliquid provided to CRACs 116 and CRACs 116 may be chilled water airconditioning systems having chilled water valves. The chilled watervalves may be on/off valves or be variable valves, such as capacitymodulated valves. Heat rejection device 124 may also be a refrigerationcondenser system, in which case a refrigerant is provided to CRACs 116and CRACs 116 may be phase change refrigerant air conditioning systemshaving refrigerant compressors, such as a DX system. Each CRAC 116 mayinclude a control module 125 that controls the CRAC 116.

In an aspect, CRAC 116 includes a variable capacity compressor and mayfor example include a variable capacity compressor for each DX coolingcircuit of CRAC 116. It should be understood that CRAC 116 may, as isoften the case, have multiple DX cooling circuits. In an aspect, CRAC116 includes a capacity modulated compressor type of compressor or a4-step semi-hermetic compressor, such as those available from EmersonClimate Technologies, Liebert Corporation or the Carlyle division ofUnited Technologies. CRAC 116 may also include one or more air movingunits 119, such as fans or blowers. The air moving units 119 may beprovided in CRACs 116 or may additionally or alternatively be providedin supply air plenum 114 as shown in phantom at 121. Air moving units119, 121 may illustratively have variable speed drives.

A typical CRAC 200 having a typical chilled water cooling circuit isshown in FIG. 2. CRAC 200 has a cabinet 202 in which a cooling coil 204is disposed. Cooling coil 204 may be a V-coil. Cooling coil 204 may alsobe an A-coil, or a coil having an inclined slab configuration. An airmoving unit 206, such as a fan or squirrel cage blower, is also disposedin cabinet 202 and situated to draw air through cooling coil 204 from aninlet (not shown) of cabinet 202, where it is cooled by cooling coil204, and directs the cooled air out of an outlet 208, which may be aplenum. Cooling coil 204 is coupled to a source of chilled water 210,such as a water chiller, by pipes 212 so that chilled water iscirculated through cooling coil 204.

A typical CRAC 300 having a typical DX cooling circuit is shown in FIG.3. CRAC 300 has a cabinet 302 in which a cooling coil 304 is disposed.Cooling coil 304 is typically an evaporator coil, and may be a V-coil.Cooling coil 304 may also be an A-coil, or a coil having an inclinedslab configuration. An air moving unit 306, such as a fan or squirrelcage blower, is also disposed in cabinet 302 and situated to draw airthrough cooling coil 304 from an inlet (not shown) of cabinet 302, whereit is cooled by cooling coil 304, and direct the cooled air out of anoutlet 308, which may be a plenum. Cooling coil 304, a compressor 310, acondenser 312 and an expansion valve 314 are coupled together in knownfashion in a DX refrigeration circuit. A phase change refrigerant iscirculated by compressor 310 through condenser 312, expansion valve 314,cooling coil 304 and back to compressor 304. Condenser 312 may be any ofa variety of types of condensers conventionally used in cooling systems,such as an air cooled condenser, a water cooled condenser, or glycolcooled condenser. Compressor 310 may be any of a variety of types ofcompressors conventionally used in DX refrigeration systems, such as ascroll compressor. When cooling coil 304 is a V-coil or A-Coil, ittypically has a cooling slab on each leg of the V or A, as applicable.Each cooling slab may, for example, be in a separate cooling circuitwith each cooling circuit having a separate compressor. In this regard,CRAC 300 would then have multiple compressors 310. Alternatively, thefluid circuits in each slab such as where there are two slabs and twocompressor circuits, can be intermingled among two compressor circuits.

Cooling coils 204, 304 are typically fin-and-tube evaporator coils andare used to both cool and dehumidify the air passing through them.Typically, CRAC's such as CRAC's 200, 300 are designed so that thesensible heat ratio (“SHR”) is typically between 0.85 to 0.95.

A system known as the GLYCOOL free-cooling system is available fromLiebert Corporation of Columbus, Ohio. In this system, a second coolingcoil, known as a “free cooling coil,” is added to a CRAC having a normalglycol system. This second coil is added in the air stream ahead of theupstream cooling coil. During colder months, the glycol solutionreturning from the outdoor drycooler is routed to the second coolingcoil and becomes the primary source of cooling to the data center. Atambient temperatures below 35 deg. F. (Fahrenheit), the cooling capacityof the second cooling coil is sufficient to handle the total coolingneeds of the data center and substantially reduces energy costs sincethe compressor of the CRAC need not be run. The second or free coolingcoil does not provide 100% sensible cooling and has an airside pressuredrop similar to the downstream cooling coil.

Server temperature deltas have been increasing, and in some cases haveincreased from the 10-20 deg. F. range to over 30 deg. F. A servertemperature delta is the difference between the inlet and outlet (orexhaust) temperatures of the air circulated through the server to coolit. This increase in server temperature deltas has in turn increased thetemperature difference across the CRACs. The temperature differenceacross the CRAC is the difference in temperature between the air beingdrawn into the cooling coil of the CRAC and the cooled air exiting theCRAC. The temperature difference across a typical chilled water CRAC anda typical DX CRAC is about 20 deg. F. If the temperature differenceacross the CRAC is less than the server temperature delta, then the airflow to the server must be increased to provide the requisite coolingfor the server. This will generate excessive air flow bypass that willreturn to the CRAC, wasting some of the cooling. Additionally, as serverloads have increased, the proportion of sensible heat load in the datacenter has increased compared to the latent heat load, thus increasingthe sensible heat ratio (SHR) requirements.

SUMMARY

This section provides a general summary of the disclosure, and is not acomprehensive disclosure of its full scope or all of its features.

In accordance with an aspect of the present disclosure, a computer roomair conditioner (“CRAC”) has a cabinet having an air inlet through whichreturn air from an area is drawn and an air outlet through which aircooled by the CRAC is exhausted. An air moving unit is disposed in thecabinet as are a plurality of cooling coils, which are in separatecooling circuits. The cooling coils are arranged so that the air passesthrough the cooling coils in serial fashion, that is, first through anupstream cooling coil and then through a downstream cooling cool. Ifthere are more than two cooling circuits, then the air passes in turnthrough each subsequent downstream cooling coil of each subsequentdownstream cooling circuit. Each upstream cooling coil acts as apre-cooler to the subsequent downstream cooling coil. The CRAC includesa controller that controls the cooling provided by the cooling circuits.The controller controls the cooling provided by at least the mostupstream cooling circuit so that it provides only sensible cooling. Thatis, the cooling provided by the most upstream cooling circuit iscontrolled so that is it provides one hundred percent sensible cooling.In an aspect, the most upstream cooling circuit is used to provide allthe cooling.

In an aspect, the most downstream cooling circuit is controlled toprovide any additional sensible cooling that may be needed as well asany latent (dehumidification) that may be needed and all the coolingcircuits upstream of the most downstream cooling circuit are controlledto provide only sensible cooling. In an aspect, the most downstreamcooling circuit is used to provide all the cooling and is controlled toprovide sensible cooling and such latent cooling that may be needed.

In an aspect, at least the most upstream cooling coil is a microchannelcooling coil and at least the most downstream cooling coil is afin-and-tube cooling coil.

In an aspect, the cooling coils are fin-and-tube cooling coils.

In an aspect, the cooling coils are microchannel cooling coils.

In an aspect, the cooling coil of the most upstream cooling circuit ispositioned at the air inlet of the CRAC. In aspect, the cooling coil ofthe most upstream cooling circuit is positioned at an inlet of thecooling coil of the next downstream cooling circuit.

In an aspect, the most upstream cooling circuit is a pumped refrigerantcooling circuit. In an aspect, the most upstream cooling circuit is achilled water cooling circuit. In an aspect, the most upstream coolingcircuit is a DX cooling circuit. In an aspect, the most downstreamdownstage cooling circuit is a DX cooling circuit. In an aspect, themost downstream cooling circuit is a chilled water cooling circuit. Inan aspect, the most downstream downstage cooling circuit is a pumpedrefrigerant cooling circuit.

In an aspect, the downstream cooling coil is disposed in the cabinet andthe upstream cooling coil is in an air inlet plenum outside the cabinetwhich is coupled to the air inlet of the cabinet.

In an aspect, the upstream cooling circuit cools air passing through theupstream cooling coil sufficiently so that a cooling capacity of thedownstream cooling circuit is sufficient to reduce a temperature of theair passing through the downstream cooling to below a dew point of theair to provide latent cooling.

In an aspect, the upstream cooling circuit increases a temperature deltaacross the computer room air conditioner by at least ten degreesFahrenheit.

Further areas of applicability will become apparent from the descriptionprovided herein. The description and specific examples in this summaryare intended for purposes of illustration only and are not intended tolimit the scope of the present disclosure.

DRAWINGS

The drawings described herein are for illustrative purposes only ofselected embodiments and not all possible implementations, and are notintended to limit the scope of the present disclosure.

FIG. 1 is a schematic illustrating a prior art data center;

FIG. 2 is a simplified perspective view of a prior art CRAC having achilled water cooling circuit;

FIG. 3 is a simplified perspective view of a prior art CRAC having a DXcooling circuit;

FIG. 4 is a schematic of a CRAC in accordance with an aspect of thepresent disclosure with an upstream cooling circuit and a downstreamcooling circuit;

FIG. 5 is a simplified perspective view of a CRAC having the coolingcircuits of the CRAC of FIG. 4 and

FIG. 6 is a schematic of a CRAC in accordance with an aspect of thepresent disclosure with two upstream cooling circuits and a downstreamcooling circuit.

Corresponding reference numerals indicate corresponding parts throughoutthe several views of the drawings.

DETAILED DESCRIPTION

Example embodiments will now be described more fully with reference tothe accompanying drawings.

FIG. 4 is a simplified schematic of a CRAC 400 having an upstreamcooling circuit 401 and a downstream cooling circuit 402 in accordancewith an aspect of the present disclosure. It should be understood thatcooling circuits 401, 402 are illustratively separate cooling circuits.In the embodiment of FIG. 4, upstream cooling circuit 401 is a pumpedrefrigerant cooling circuit and downstream cooling circuit 402 is a DXrefrigeration circuit. Upstream cooling circuit 401 includes a coolingcoil 404 (sometimes referred to herein as upstream cooling coil 404),flow regulator 406, condenser 408 and pump 410 arranged in a pumpedrefrigerant cooling circuit such as is disclosed in U.S. patentapplication Ser. No. 10/904,889 owned by the owner of the presentapplication and the entire disclosure of which is incorporated herein byreference. Downstream cooling circuit 402 includes a cooling coil 412(sometimes referred to herein as downstream cooling coil 412), expansionvalve 414, condenser 416 and compressor 418 arranged in a conventionalDX refrigeration circuit. In the pumped refrigerant cooling circuitillustratively used for upstream cooling circuit 401, pump 410circulates a phase change refrigerant throughout upstream coolingcircuit 401 by pumping it so that it flows through flow regulator 406,upstream cooling coil 404 (which is illustratively an evaporator),through condenser 408 and back to pump 410. Pumping the refrigerant withpump 410 increases the pressure of the refrigerant but withoutappreciably increasing its enthalpy. In the DX cooling circuitillustratively used for downstream cooling circuit 402, a phase changerefrigerant is circulated by the compressor 418 so that it flows fromthe compressor 418, through the condenser 416, expansion valve 414,downstream cooling coil 412 (which is illustratively an evaporator) andback to the compressor 418. The cooling coils 404, 412 of upstream anddownstream cooling circuits 401, 402 are arranged so that air drawn inthrough an inlet of the CRAC flows in serial fashion through coolingcoils 404, 412, that is, the air flows first through the upstreamcooling coil 404 of upstream cooling circuit 401 and then through thedownstream cooling coil 412 of the downstream cooling circuit 402. Inthis regard, upstream cooling circuit 401 is a pre-cooling circuit inthat upstream cooling coil 404 cools air before the air flows throughthe downstream cooling coil 412 of the downstream cooling circuit 402.CRAC 400 includes an air moving unit, such as air moving unit 508 (FIG.5) which may illustratively be a fan or blower, that pulls the air intoCRAC 400, through cooling coils 404, 412, and out an air outlet of CRAC400.

It should be understood that upstream cooling circuit 401 can be otherthan a pumped refrigerant cooling circuit, such as a chilled watercooling circuit or a DX refrigeration circuit. It should be understoodthat downstream cooling circuit 402 can be other than a DX refrigerationcircuit, such as a chilled water cooling circuit or a pumped refrigerantcooling circuit.

Condenser 408 of upstream cooling circuit 401 may in particularpreferably be a building chilled water heat rejection device asdescribed above with regard to heat rejection device 124 of FIG. 1.However, it should be understood that condensers 408, 416 can be any ofthe heat rejection devices described above with regard to heat rejectiondevice 124 of FIG. 1.

CRAC 400 includes a controller 420 that controls cooling circuits 401,402. Controller 420 controls upstream cooling circuit 401 so that itprovides one-hundred percent sensible cooling. It does so by controllingthe temperature of the cooling fluid (such as a phase changerefrigerant) flowing in upstream cooling circuit 401 so that when itpasses through upstream cooling coil 404 of upstream cooling circuit401, the temperature of the refrigerant is above the dew point of theair flowing through upstream cooling coil 404. The air flowing throughupstream cooling coil 404 is typically the return air from the areabeing cooled by CRAC 400 that is drawn into CRAC 400 through the returnair inlet of CRAC 400. In an aspect, controller 420 controls downstreamcooling circuit 402 to provide any additional sensible cooling that maybe needed as well as any latent cooling that may be needed. In somecases, upstream cooling circuit 401 can provide all the sensible coolingrequired and if no latent cooling is required, upstream cooling circuit401 then provides all the cooling and controller 420 controls upstreamand downstream cooling circuits 401, 402 so that upstream coolingcircuit provides all the cooling, which is only sensible cooling. Inother cases, downstream cooling circuit 402 provides additional sensiblecooling and/or latent cooling, depending on whether additional sensiblecooling is needed, whether latent cooling is needed, or whether bothlatent cooling and additional sensible cooling are needed and iscontrolled accordingly by controller 420. In yet other cases, such aswhere the total cooling load is light and latent cooling is needed, thedownstream cooling circuit 402 can be used to provide all the coolingand is controlled accordingly by controller 420 which also controlsupstream cooling circuit 401 so that it is not providing cooling.

In an aspect, upstream cooling coil 404 of upstream cooling circuit 401is a microchannel cooling coil. Upstream cooling coil 404 mayillustratively be a microchannel heat exchanger of the type described inU.S. Ser. No. 12/388,102 filed Feb. 18, 2009 for “Laminated Manifold forMicrochannel Heat Exchanger” the entire disclosure of which isincorporated herein by reference. Upstream cooling coil 404 mayillustratively be a MCHX microchannel heat exchanger available fromLiebert Corporation of Columbus, Ohio. While one advantage of using amicrochannel cooling coil for cooling coil 404 of upstream coolingcircuit 401 is that microchannel cooling coils have air side pressuredrops across them that are significantly less than fin-and-tube coolingcoils having comparable cooling capacity, it should be understood thatcooling coil 404 can be other than a microchannel cooling coil, and mayfor example be a fin-and-tube cooling coil.

In an aspect, downstream cooling coil 412 of downstream cooling circuit402 is a fin-and-tube cooling coil. It should be understood, however,that downstream cooling coil 412 can be other than a fin-and-tubecooling coil, and may for example be a microchannel cooling coil. Inthis case, both the upstream and downstream cooling circuits 401, 402are operated to provide sensible cooling only.

FIG. 5 shows an illustrative embodiment of CRAC 400. CRAC 400 includes acabinet 500 having a return air inlet 502 and an air outlet 504, such asa plenum. An air filter 506 is disposed at return air inlet 502 so thatair flowing into CRAC 400 through return air inlet 502 flows through airfilter 506 before flowing through the rest of CRAC 400.

In the embodiment shown in FIG. 5, downstream cooling coil 412 ofdownstream cooling circuit 402 is an A-coil and is disposed in cabinet500 between return air inlet 502 and air outlet 504. Downstream coolingcoil 412 thus has a cooling slab 510 on each leg of the A. An air movingunit 508, such as a fan or squirrel cage blower, is disposed in cabinet500 between a downstream side of downstream cooling coil 412 and airoutlet 504. Upstream cooling coil 404 of upstream cooling circuit 401may illustratively be disposed in cabinet 500 at return air inlet 502,preferably after air filter 506. It should be understood that upstreamcooling coil 404 could be positioned in cabinet 500 anywhere betweenreturn air inlet 502 and downstream cooling coil 412. It should beunderstood that upstream cooling coil 404 could be single cooling slab,or could be segmented into multiple cooling slabs, as could downstreamcooling coil 412. It should also be understood that upstream coolingcoil 404 could be configured as a rectangular cooling slab, as shown inFIG. 5, or could be configured as an A-coil similar to the configurationof downstream cooling coil 412. Upstream and downstream cooling coils404, 412 could also be V-coils. Upstream cooling coil 404 mayillustratively be configured so that when it is positioned in cabinet500, it has a sealing configuration so that air flowing from air inlet502 to downstream cooling coil 412 of downstream cooling circuit 402must flow through upstream cooling coil 404. In the embodiment shown inFIG. 5, air moving unit 508 is utilized to draw air through bothupstream cooling coil 404 of upstream cooling circuit 401 and downstreamcooling coil 412 of downstream cooling circuit 402.

It should be understood that upstream cooling coil could be disposedinside an air inlet plenum 512 outside of cabinet 500 that is coupled toreturn air inlet 502 of cabinet 500, as shown in phantom in FIG. 5 withthe upstream cooling coil designated by reference number 404′. Thisconfiguration may illustratively be utilized when adding upstreamcooling circuit 401 to a CRAC, such as retrofitting a CRAC to addupstream cooling circuit 401.

By providing upstream cooling circuit 401 with cooling coil 404 thatpre-cools the air before it flows into cooling coil 412 of downstreamcooling circuit 402, the maximum temperature delta of CRAC 400 can beincreased thus increasing the cooling capacity of CRAC 400. For example,a typical CRAC having a DX refrigeration circuit may have a maximumtemperature delta of about 20 deg. F. Upstream cooling circuit 401 withcooling coil 404 may illustratively be configured to add an additionalten deg. F of temperature delta across the CRAC, increasing the maximumtemperature delta across the CRAC to about thirty deg. F.

Upstream cooling circuit 401 may be a retrofit kit for existing CRACs,or may be installed during the manufacture of the CRAC. In this regard,by adding upstream cooling circuit 401 to a CRAC that otherwise isunable to provide latent cooling to dehumidify the air, the coolingcircuit of the CRAC, which will then be a downstream cooling circuit,may then be able to provide latent cooling. For example, the temperatureof the air entering a CRAC may be sufficiently high that the coolingcircuit of the CRAC is not able to provide sufficient cooling to reducethe temperature of the air as it passes through the cooling cool of thiscooling circuit to below the dew point, and thus CRAC is not able toprovide latent cooling. By adding upstream cooling circuit 401, the airis pre-cooled before it reaches the cooling coil of the cooling circuitof the CRAC, which is now a downstream cooling circuit. Since thetemperature of the air entering the cooling coil of the downstreamcooling circuit has been lowered, the downstream cooling circuit thenhas sufficient cooling capacity to reduce the temperature of the airpassing through its cooling coil to below a dewpoint of the air and canthus provide latent cooling. In this regard, upstream cooling circuit401 cools the air passing through upstream cooling coil 404 sufficientlyso that a cooling capacity of downstream cooling circuit 402 issufficient to reduce a temperature of the air passing through downstreamcooling coil 408 to below a dew point of the air to provide latentcooling.

In a an aspect, a CRAC with both upstream cooling circuit 401 anddownstream cooling circuit 402 can be optimally controlled by controller420 to use the most efficient of the upstream cooling circuit 401 anddownstream cooling circuit 402 based on heat load and environmentalconditions.

It should be understood that the CRAC can have more than two coolingcircuits. FIG. 6 shows a CRAC 600 having three cooling circuits 602,604, 606. Cooling circuits 602, 604 may illustratively be pumpedrefrigerant cooling circuits having the same components as coolingcircuit 401 (FIG. 4) and cooling circuit 606 may illustratively be adirect expansion cooling circuit having the same components as coolingcircuit 402 (FIG. 4). Cooling circuit 602 is the most upstream coolingcircuit having its cooling coil 404 disposed in the most upstreamposition, cooling circuit 606 is the most downstream cooling circuithaving its cooling coil 412 disposed in the most downstream location,and cooling circuit 604 has its cooling coil 404 disposed downstream ofcooling coil 404 of most upstream cooling circuit 602 and upstream ofcooling coil 412 of most downstream cooling circuit 606.

Spatially relative terms, such as “inner,” “outer,” “beneath”, “below”,“lower”, “above”, “upper” and the like, may be used herein for ease ofdescription to describe one element or feature's relationship to anotherelement(s) or feature(s) as illustrated in the figures. Spatiallyrelative terms may be intended to encompass different orientations ofthe device in use or operation in addition to the orientation depictedin the figures. For example, if the device in the figures is turnedover, elements described as “below” or “beneath” other elements orfeatures would then be oriented “above” the other elements or features.Thus, the example term “below” can encompass both an orientation ofabove and below. The device may be otherwise oriented (rotated 90degrees or at other orientations) and the spatially relative descriptorsused herein interpreted accordingly.

The foregoing description of the embodiments has been provided forpurposes of illustration and description. It is not intended to beexhaustive or to limit the invention. Individual elements or features ofa particular embodiment are generally not limited to that particularembodiment, but, where applicable, are interchangeable and can be usedin a selected embodiment, even if not specifically shown or described.The same may also be varied in many ways. Such variations are not to beregarded as a departure from the invention, and all such modificationsare intended to be included within the scope of the invention.

1. A computer room air conditioner, comprising: a cabinet having an airinlet and an air outlet; an air moving unit disposed in the cabinet; aplurality of separate cooling circuits including an upstream coolingcircuit and a downstream cooling circuit; the upstream cooling circuitincluding a cooling coil (upstream cooling coil) and the downstreamcooling circuit including a cooling coil (downstream cooling coil)wherein the cooling coils are arranged so that air to be cooled passesthrough them in serial fashion, first through the upstream cooling coiland then through the downstream cooling coil wherein the upstreamcooling cool is a pre-cooler to the downstream cooling coil; and acontroller that controls the cooling circuits, the controllercontrolling the upstream cooling circuit when the upstream coolingcircuit is being used to provide cooling to provide only sensiblecooling.
 2. The computer room air conditioner of claim 1, wherein thecontroller controls the downstream cooling circuit to provide any latentcooling that is required and any sensible cooling that is required inaddition to the sensible cooling provided by the upstream coolingcircuit.
 3. The computer room air conditioner of claim 1 wherein thecontroller controls the downstream cooling circuit to provide sensiblecooling and any latent cooling that is required and controls theupstream cooling circuit so that it is not providing any cooling.
 4. Thecomputer room air conditioner of claim 1 wherein the upstream coolingcool is a microchannel cooling coil and the downstream cooling coil is afin-and-tube cooling coil.
 5. The computer room air conditioner of claim1 wherein the upstream cooling coil and the downstream cooling coil areboth microchannel cooling coils.
 6. The computer room air conditioner ofclaim 1 wherein the upstream cooling coil and the downstream coolingcoils are both fin-and-tube cooling coils.
 7. The computer room airconditioner of claim 1 wherein the upstream cooling circuit is a pumpedrefrigerant cooling circuit having the upstream cooling coil, a flowregulator, a condenser and a pump arranged in the pumped refrigerantcooling circuit, and the downstream cooling circuit is a directexpansion cooling circuit having the downstream cooling coil, anexpansion valve, a condenser and a compressor arranged in the directexpansion cooling circuit.
 8. The computer room air conditioner of claim1 wherein the upstream cooling coil is disposed at the air inlet of thecabinet.
 9. The computer room air conditioner of claim 1 wherein theupstream cooling coil is disposed at an air inlet of the downstreamcooling coil.
 10. The computer room air conditioner of claim 1 includingat least three cooling circuits, the controller controlling all but amost downstream cooling circuit to provide only sensible cooling whenthese cooling circuits are being used to provide cooling.
 11. Thecomputer room air conditioner of claim 10 wherein the controllercontrols controlling the most downstream cooling circuit to provide anylatent cooling that is required and any sensible cooling that isrequired in addition to the sensible cooling provided by the coolingcircuits upstream of the most downstream cooling circuit.
 12. Thecomputer room air conditioner of claim 10 wherein the controllercontrols the downstream cooling circuit to provide sensible cooling andany latent cooling that is required and controls the upstream coolingcircuit so that it is not providing any cooling.
 13. The computer roomair conditioner of claim 1 wherein the upstream and downstream coolingcoils are disposed in the cabinet.
 14. The computer room air conditionerof claim 1 wherein the downstream cooling coil is disposed in thecabinet and the upstream cooling coil is in an air inlet plenum outsidethe cabinet which is coupled to the air inlet of the cabinet.
 15. Thecomputer room air conditioner of claim 14 wherein the upstream coolingcircuit cools air passing through the upstream cooling coil sufficientlyso that a cooling capacity of the downstream cooling circuit issufficient to reduce a temperature of the air passing through thedownstream cooling to below a dew point of the air to provide latentcooling.
 16. The computer room air conditioner of claim 1 wherein theupstream cooling circuit increases a temperature delta across thecomputer room air conditioner by at least ten degrees Fahrenheit.